It is a common practice to planarize not only the intrinsic surface of a semiconductor but also the surface of a processed layer formed thereon. For building up, for example, a semiconductor IC having multiple circuit layers in which wiring layers are arranged three-dimensionally, it is necessary to planarize the surface of the interlayer insulator film (silicon oxide film). Namely, when an aluminum wiring circuit is formed as the first layer (the bottom layer) and a silicon oxide film is applied thereto by, for example, CVD, a surface irregularity is brought about on the silicon oxide film due to the presence of the underlying aluminum wiring. If a photolithography or a dry-etching is carried out on this irregular surface oxide film for forming a second layer of a aluminum circuit, failures, such as defocussing in the exposed resist patterning and wiring discontinuity due to dry-etching at the staged portion of the irregular surface, may occur.
Accordingly, a practice of polishing such surfaces with an abrasive is incorporated. This practice comprises polishing the surface on a semiconductor substrate using a polishing element, such as a polishing pad, to planarize the surface while supplying an abrasive in a form of a slurry in between the substrate surface and the polishing pad with dilution by water supplied thereto.
As the abrasive, colloidal silica has found its application thereto due to its high dispersibility, uniform particle size around the average size and other conveniences, wherein it is used in a form of colloidal silica suspension by dispersing it in a dispersion medium, such as water.
When polishing is effected using such an abrasive, the polished material on the surface of the semiconductor substrate is scraped off and, at the same time, the particles of the colloidal silica may also suffer a crushing to form a polishing refuse. This polishing refuse itself reduces the polishing ability of the abrasive. Occasionally, there may occur a formation of gelled product in the suspension during the polishing operation due to a partial drying of the abrasive suspension. In general, a spent abrasive suspension is discarded as such, since a large-sized polishing refuse and aggregation may cause damage on the polished surface and since the polishing ability is decreased by accumulation of polishing refuse.
In Japanese Patent Application Kokai Hei-2-25762, there is proposed a method for separating large-sized particles from a spent polishing liquor by centrifugation and to reuse the remaining liquor after adjusting the colloidal silica concentration by adding thereto water or additional colloidal silica, in order to overcome the situation that the existing technique for reclaiming the spent abrasive suspension using a conventional filter did not attain a sufficient removal of the particles that cause damage to the polished surface.
However, this method requires a centrifugal force of 8,000 G or higher due to the small mass difference between the particles to be removed and the colloidal silica particles, necessitating a large and complicated apparatus and resulting in a difficulty in the operation and maintenance of the apparatus. This method also exhibited a problem in that some reduction in the polishing performance is unavoidable due to the accumulation of materials which reduce the polishing ability, such as micronous polishing refuse, since the method provides only for the removal of particles of greater masses. This applies not only to colloidal silica but also to other abrasives.